Work vehicles employ transmissions that are quite different from those in standard passenger cars. In work vehicles, the power to weight ratio is typically much more limited than it is in automobiles. This means that the engines powering work vehicles are usually running at or near capacity much of the time.
As a result, and particularly for ground engaging vehicles such as tractors towing plows, for example, shifting is critical. In work vehicles, shifting must occur rapidly and with very little delay from the time power is disengaged from the drive wheels to the time power is reengaged to the drive wheels. This rapid disengagement and reengagement permits work vehicles to change gears much more rapidly than is possible in standard automobiles.
Work vehicles are also provided with gearboxes having much closer gear ratios. Unlike a typical automobile, which may have four or even five forward speeds, work vehicles typically have 8, 10 or even 20 forward gears. With this many forward gears, work vehicles spend a great deal of time shifting from one gear ratio to another to optimize vehicle speed and engine load. Unlike automotive transmissions, which have a single large master clutch to disengage the drive train, and three or four shifting forks to shift the gears on their shafts, work vehicle transmissions put several small clutches inside the transmission disposed between shafts and gears and between gears themselves.
In power shift transmissions, there are piston assemblies that facilitate actuation of the clutch and engagement of the clutch plates in the clutch stack, which rotate during operation. A plurality of clutch assemblies and their corresponding piston assemblies engage the different gear ratios. The rotation of the clutch assemblies creates a build up of pressure or “centrifugal head” in the clutch assemblies. Springs, such as arrangements of Belleville washers, are utilized to keep the piston assemblies from engaging the clutch assemblies. The force of the spring provides sufficient force on the piston assembly that centrifugal head is substantially prevented from engaging the clutch during normal operation. If the piston assembly unintentionally engages the clutch assembly when it is not supposed to, the clutch assembly will burn up or wear out.
However, there are times when the transmission is operated at higher speeds than designed for, (e.g., accelerating down hill). This higher rotational speed will turn the clutch and piston assemblies at a speed that will generate excessive centrifugal head behind the piston assembly, causing the pistons to engage or partially engage the clutch assembly. The contact within the clutch assembly of the clutch plates during the times of higher rotation (i.e., unintentional engagement of the clutch assembly) may cause a premature transmission failure.
A known clutch assembly design includes a piston with inner diameter and outer diameter seal rings which are located in a bore to prevent pressurized hydraulic oil from escaping to create a pressure build up behind the piston when engaged. The outer diameter bore is typically a single constant diameter and the seal is typically a solid seal with an o-ring underneath to energize the seal outward against the bore. The contact of seal against the bore will seal the piston pressure cavity and not allow any leakage.
To reduce the effects of centrifugal head build-up in the clutches, several methods have been used. The spring force against the piston assembly can be increased to compensate for the higher speeds. This will, however, reduce the clutch capacity when the clutch is engaged. Another method has been to add a relief valve in the piston or clutch assembly, to reduce the pressure in a controlled manner. The introduction of a relief valve suffers from the drawback that the relief valve will add significant additional cost, is subject to potential malfunction, and may not eliminate all the centrifugal head pressure.
What is needed, therefore, is a clutch assembly that reduces centrifugal head and reduces or eliminates forces urging engagement or partial engagement of the non-activated clutch assemblies during times of increased rotational speed of the clutch assembly. These and other advantages are provided by the clutch assembly described herein.